The geographical area covered by a typical cellular mobile systems is divided into smaller radio coverage areas, cells, to achieve a more efficient utilization of the frequency band through frequency re-use. The size of cells varies from a mobile system to another as well as geographically within a mobile system, depending on a multitude of factors, such as the capacity required (number of channels, terrain, transmitting power levels, etc.). One factor which influences cell size is the maximum transmitting power of a mobile station. The mobile stations in conventional mobile systems were vehicle-mounted high-power radio stations. In addition to these, small hand-held mobile stations have emerged; these mobile stations have a considerably lower transmitting power than the vehicle-mounted stations in order to reduce power consumption, for example. These hand-held mobile stations have a considerably smaller range of operation and, as a result, the cell size must also be smaller.
In modern mobile communication systems, both low-power hand-held mobile stations and vehicle-mounted mobile stations that have a higher output power are used side by side. Further, hierarchical networks with geographically at least partly overlaying large macrocells and small microcells have been employed in the mobile communication systems. It would be advantageous if high-power mobile stations were assigned to a macrocell network, and low-power hand-held mobile stations to a microcell network. Because the high-power vehicle-mounted stations, moreover, usually move in the network with higher speed than the hand-held mobile phones, the use of macrocells results in a smaller number of handovers. The hand-held mobile stations are, in turn, relatively immobile and thus the number of handovers will not be unreasonable even in a dense microcell network. A handover between two microcells is usually made on the basis of the criteria of the radio connection, on the basis of the level or quality of the received signal, for example. The system may, in addition, include other handover types, such as a power budget handover.
A deficiency in the present-day networks which are hierarchically divided into microcells and macrocells is that there is no way of making it possible to direct mobile stations to macrocells and microcells according to their power levels, in other words, low power mobile stations to microcells, and high power mobile stations to macrocells. As a result, a mobile station may unnecessarily use a cell which fits worse to it as far as the cell size is concerned, which results in an inefficient utilization of the network capacity and may cause a considerable increase in the number of handovers.